The circuitry used to interconnect components mounted on a silicon substrate is more susceptible to faults as the number of components and density of circuit traces fabricated on the substrate increases. This problem is particularly significant in the fabrication of multi-chip modules formed on a single IC substrate. Any significant defect that occurs during the fabrication of an IC will normally result in it being discarded as usuable. To minimize fatal defects, IC circuit designers often include redundant components on the silicon substrate and may even provide a redundant interconnecting conductor for each of the conductors comprising the active circuit. Clearly, this level of redundancy doubles conductor density and is generally unacceptable as requirements for circuit density increase beyond a certain level.
Alternatively, it would seem that the yield rate of high density ICs might be improved by repairing defects in the circuit. While techniques already exist for modifying an IC after it is fabricated, such modifications are generally limited to disconnection and rerouting of conductor lines at the surface of the IC, using a focused laser beam. This technique is ineffective with interconnection schemes employing multiple signal planes in which fractional mil wide conductor lines are packed at relatively high density and are fully encapsulated within the IC package.
The two most common faults in a high density IC are broken or open conductive traces and short circuits between adjacent traces. To be effective, any provisions for repairing an IC should permit shorted conductors to be disconnected and circuit paths to be rerouted around open traces--all within signal planes that are disposed well below the surface of the IC. In addition, the interconnecting circuitry of the IC should permit minor modifications to correct circuit design errors that have been implemented, e.g., to correct design errors in prototypes.
In consideration of the foregoing, it is an object of this invention to economically reroute interconnections around circuit defects and faults in an IC, without sacrificing the level of integration in the device or degrading its performance. A further object is to provide an IC that incorporates an integral facility to replace any defective conductor, but does not require a redundant conductor for each signal carrying conductor. Yet a further object is to disconnect and reroute around defective conductors by changing connections at sites that are buried within the IC. These and other objects and advantages of the present invention will be apparent from the attached drawings and by reference to the Description of the Preferred Embodiments that follows.